* Prev: <a href="/content/topics/robotics/ros_book/32_prr_ros_topics_additional/">3a: Custom Messages</a> * Next: <a href="/content/topics/robotics/ros_book/45_prr_ros_services_additional/">4a: Services - example</a> <slide_break></slide_break> ### services * Very related to a topic * Instead of "subscribing" and "publishing" * Services are a classic "client/server" * Client Node "C" asks a question of Service "S" and blocks awaiting the response * Service "S" responds immediately with a result, allowing "C" to continue * Services are good for questions that have immediate answers * They are *synchronous* whereas topics are *asynchronous* * Another way to think of a service is as a "Remote Procedure Call" <slide_break></slide_break> ### The Service Node * Declares its existence and it's name * Goes into a wait state for another node to request the service <slide_break></slide_break> ### A client Node * It's a normal node (nothing magic about it) * It includes code to send a request to the service (ask it a question) * The code is more, but the effect is of a function call * Impressive because it is inter-process and even inter-host - i.e. can be over the internet <slide_break></slide_break> ### Defining a Service inputs and outputs * Analogous to defining the "method signature" of the service * Very reminiscent of how Topic messages were defined * You will find this example in the `samples` package